The present invention relates to a method of purifying flue gas from the firing equipment of a combustion unit, especially flue gas from firing equipment to which low-ash fuel is supplied, or from a grating firing having a high degree of ash removal, with the flue gas containing noxious compounds. A finely divided or pulverous basic additive is used on which the noxious compounds produced during firing are adsorbed. Flue dust, especially flue dust/additive mixture, that is carried along in the flue gas is separated from the flue gas, with at least a portion of the separated-off flue dust being hydrated and sifted, whereby subsequently at least a portion of the dry, hydrated flue dust is returned to the flue gas purification process.
Methods where the noxious gases are bound by a basic additive in the region of the firing equipment of a combustion unit, such as a steam generator, are classified as primary flue gas purification methods. In addition to sulfur compounds, halogen compounds such as HC1 and HF can be present in the flue gas as noxious gases. In the context of the present application, the term flue dust refers to the additive originating from the flue gas purification, with or without flue ash derived from the fuel. Examples of additive include hydroxide or carbonate compounds of the metals calcium or magnesium, as well as mixtures thereof, with limestone being preferred. The main problem of the heretofore known methods is that the additive can be used only partially. One criterium that is to be observed in this connection is the reaction temperature at which the noxious gases can be bound. Depending upon the time and temperature characteristics of the additive that is used, this reaction temperature should, if possible, be no greater than 1250.degree. C. in order to prevent not only sintering of the surface-active additive, but also encapsulation of the additive by flue ash resulting from the firing. Another criterium is the effective retention time of the additive in the flue gas stream from which the noxious compounds are to be removed. This retention time is particularly greatly affected by the mixing and distribution of the additive in the flue gas, as well as by the length of time the additive remains in the combustion unit. All improvements to the method therefore contain measures directed toward increasing the utilization of the additive, and hence also the degree of purification.
German Offenlegungsschrift No. 34 28 502 discloses a method for dry desulfurization of flue gases that contain flue ash, especially flue gases from a steam generator that is fired with brown coal or lignite. According to this method, additive in the form of limestone dust is supplied in the region of the firing, is separated off as flue dust after a certain retention time in the flue gas stream together with the accompanying flue ash, and subsequently, after being treated with dry steam, is returned to the already-cooled flue gas stream. During the steam treatment, the additive is reduced in size by a steam stream in the lower portion of a fluidized bed that is provided with two oppositely directed discharge nozzles, similar to a jet mill; this provides a new active surface at the core of the additive. At the same time, non-converted, free calcium oxide is hydrated by the steam. However, these measures are defined by the speed of the exothermic hydration, by the state or composition of the separated-off flue ash/additive mixture, as well as by the water/calcium oxide ratio. Thus, for example, the agglomeration of the additive particles increases when the water/calcium oxide ratio decreases. However, agglomeration counteracts the formation of new surfaces resulting from a reduction in size of the additive particles, as a result of which a desired improvement of the method is effectively limited to an increase of the retention time by return of the treated flue ash/additive mixture into the cooled flue gas stream. Furthermore, the use of dry steam which moreover is not supposed to condense pursuant to the known method, affects the energy balance of the method.
It is therefore an object of the present invention to still further improve the heretofore known method to achieve a greater retention time and utilization of the additive in the flue gas stream that is to be purified, and to achieve a greater degree of purification while avoiding the aforementioned problems.